1. Field of the Invention
This invention relates to modulators.
2. Description of the Prior Art
Amplitude modulation is realized, most commonly, with circuits which control the amplitude of a carrier signal. Amplifier circuits responsive to such modulators require good linearity if distortions and intermodulation products are to be avoided, and this becomes increasingly difficult with increased desired amplifier efficiency and power output.
To combat the problems associated with conventional amplitude modulation followed by amplification, a number of artisans have developed different techniques for obtaining high power amplitude modulated signals.
L. R. Kahn, for example, in "Comparison of Linear Single-Sideband Transmitters with Envelope Elimination and Restoration Single-Side-Band Transmitters," Proceedings of the IRE, Dec. 1956, page 1706 et seq, describes a technique whereby the modulated signal is separated into a carrier signal and an envelope signal. The two signals are amplified separately and modulation is executed at the last stage, following the power amplification.
M. I. Jacobs in U.S. Pat. No. 3,248,663, issued Apr. 26, 1966, describes a method for realizing amplitude modulation by means of phase modulation. D. C. Cox in U.S. Pat. No. 3,777,275, issued Dec. 4, 1973, expanded on the teachings of Jacobs by describing an interferometric amplifier where amplitude modulation is obtained by means of equal and constant amplitude carrier signals that are phase modulated with equal but oppositely signed angles. Amplified replicas of the phase modulated constant amplitude signals are combined, or interfered, to form the amplitude modulated signal. The Cox approach permits the use of constant modulus amplifiers which need not be totally free of distortions. The phase modulation angles of Cox are related to the input signal through an arc-sin function, which is developed by Cox through feedback means.
In an interferometric amplifier described in my copending application filed on even date herewith, it is necessary to modulate the phase of a carrier signal with an angle whose cosine is related to an input signal. That is, it is necessary to develop the signals cos(.omega.t+.phi.) and cos(.omega.t- .phi.) where .phi. equals cos.sup.-1 [ a(t)cos .alpha.t] and a(t)cos .alpha.t is the baseband signal.
Higher frequency response is realized in my interferometric amplifier when the arc-cosine function is developed with minimum or no feedback.